support dual launch templates: spot for normal builds, on-demand for nested virtualization

AWS does not allow cpu_options.nested_virtualization with spot instances. Add a second
launch template (on-demand, cpu_options enabled) alongside the existing spot template.
The autoscaler selects the template per-system based on nested_virtualization config.

- RuntimeAdapter.launch_spot -> launch_instance(nested_virtualization=False)
- EC2Runtime: selects spot or on-demand LT; raises misconfiguration error if
  on_demand_launch_template_id is empty when nested_virtualization=True
- AwsConfig: add on_demand_launch_template_id field
- SystemConfig: add nested_virtualization field
- Scheduler: looks up system config to pass nested_virtualization flag
- NixOS module: new aws.onDemandLaunchTemplateIdFile + capacity.nestedVirtualization
  options; assertion prevents enabling nestedVirtualization without the LT ID file

agent/nix_builder_autoscaler/config.py

@@ -23,6 +23,7 @@ class AwsConfig:
 
     region: str = "us-east-1"
     launch_template_id: str = ""
+    on_demand_launch_template_id: str = ""
     subnet_ids: list[str] = field(default_factory=list)
     security_group_ids: list[str] = field(default_factory=list)
     instance_profile_arn: str = ""
@@ -51,6 +52,7 @@ class SystemConfig:
     launch_batch_size: int = 1
     scale_down_idle_seconds: int = 900
     termination_cooldown_seconds: int = 180
+    nested_virtualization: bool = False
 
 
 @dataclass

agent/nix_builder_autoscaler/logging.py

@@ -12,7 +12,17 @@ from typing import Any
 class JSONFormatter(logging.Formatter):
     """Format log records as single-line JSON."""
 
-    EXTRA_FIELDS = ("slot_id", "reservation_id", "instance_id", "request_id", "error", "category", "count", "ids", "idle_seconds")
+    EXTRA_FIELDS = (
+        "slot_id",
+        "reservation_id",
+        "instance_id",
+        "request_id",
+        "error",
+        "category",
+        "count",
+        "ids",
+        "idle_seconds",
+    )
 
     def format(self, record: logging.LogRecord) -> str:
         """Format a log record as JSON."""

agent/nix_builder_autoscaler/runtime/base.py

@@ -21,8 +21,14 @@ class RuntimeAdapter(ABC):
     """Interface for compute runtime backends (EC2, fake, etc.)."""
 
     @abstractmethod
-    def launch_spot(self, slot_id: str, user_data: str) -> str:
+    def launch_instance(
-        """Launch a spot instance for slot_id. Return instance_id."""
+        self, slot_id: str, user_data: str, *, nested_virtualization: bool = False
+    ) -> str:
+        """Launch an instance for slot_id. Return instance_id.
+
+        When nested_virtualization is True, an on-demand instance is launched using
+        the on-demand launch template. When False (default), a spot instance is launched.
+        """
 
     @abstractmethod
     def describe_instance(self, instance_id: str) -> dict:

agent/nix_builder_autoscaler/runtime/ec2.py

@@ -60,6 +60,7 @@ class EC2Runtime(RuntimeAdapter):
     ) -> None:
         self._client: Any = _client or boto3.client("ec2", region_name=config.region)
         self._launch_template_id = config.launch_template_id
+        self._on_demand_launch_template_id = config.on_demand_launch_template_id
         self._subnet_ids = list(config.subnet_ids)
         self._security_group_ids = list(config.security_group_ids)
         self._instance_profile_arn = config.instance_profile_arn
@@ -67,22 +68,32 @@ class EC2Runtime(RuntimeAdapter):
         self._subnet_index = 0
         self._tailscale_socket_path = _tailscale_socket_path
 
-    def launch_spot(self, slot_id: str, user_data: str) -> str:
+    def launch_instance(
-        """Launch a spot instance for *slot_id*. Return instance ID."""
+        self, slot_id: str, user_data: str, *, nested_virtualization: bool = False
+    ) -> str:
+        """Launch an instance for *slot_id*. Return instance ID.
+
+        When nested_virtualization is True, an on-demand instance is launched using the
+        on-demand launch template (cpu_options nested virt enabled, no spot market options).
+        When False (default), a spot instance is launched using the spot launch template.
+        """
+        if nested_virtualization:
+            if not self._on_demand_launch_template_id:
+                raise RuntimeAdapterError(
+                    "nested_virtualization=True but on_demand_launch_template_id is not configured",
+                    category="misconfiguration",
+                )
+            lt_id = self._on_demand_launch_template_id
+        else:
+            lt_id = self._launch_template_id
+
         params: dict[str, Any] = {
             "MinCount": 1,
             "MaxCount": 1,
             "LaunchTemplate": {
-                "LaunchTemplateId": self._launch_template_id,
+                "LaunchTemplateId": lt_id,
                 "Version": "$Latest",
             },
-            "InstanceMarketOptions": {
-                "MarketType": "spot",
-                "SpotOptions": {
-                    "SpotInstanceType": "one-time",
-                    "InstanceInterruptionBehavior": "terminate",
-                },
-            },
             "UserData": user_data,
             "TagSpecifications": [
                 {
@@ -98,6 +109,15 @@ class EC2Runtime(RuntimeAdapter):
             ],
         }
 
+        if not nested_virtualization:
+            params["InstanceMarketOptions"] = {
+                "MarketType": "spot",
+                "SpotOptions": {
+                    "SpotInstanceType": "one-time",
+                    "InstanceInterruptionBehavior": "terminate",
+                },
+            }
+
         if self._subnet_ids:
             subnet = self._subnet_ids[self._subnet_index % len(self._subnet_ids)]
             self._subnet_index += 1

agent/nix_builder_autoscaler/runtime/fake.py

@@ -38,8 +38,10 @@ class FakeRuntime(RuntimeAdapter):
         self._tick_count: int = 0
         self._next_ip_counter: int = 1
 
-    def launch_spot(self, slot_id: str, user_data: str) -> str:
+    def launch_instance(
-        """Launch a fake spot instance."""
+        self, slot_id: str, user_data: str, *, nested_virtualization: bool = False
+    ) -> str:
+        """Launch a fake instance (nested_virtualization is accepted but ignored)."""
         if slot_id in self._launch_failures:
             self._launch_failures.discard(slot_id)
             raise RuntimeAdapterError(

agent/nix_builder_autoscaler/scheduler.py

@@ -245,8 +245,11 @@ def _launch_slot(
     """Launch a single slot. Transition to LAUNCHING on success, ERROR on failure."""
     slot_id = slot["slot_id"]
     user_data = render_userdata(slot_id)
+    system_name = slot.get("system", config.capacity.default_system)
+    sys_cfg = next((s for s in config.systems if s.name == system_name), None)
+    nested_virt = sys_cfg.nested_virtualization if sys_cfg else False
     try:
-        instance_id = runtime.launch_spot(slot_id, user_data)
+        instance_id = runtime.launch_instance(slot_id, user_data, nested_virtualization=nested_virt)
         metrics.counter("autoscaler_ec2_launch_total", {"result": "success"}, 1.0)
         db.update_slot_state(slot_id, SlotState.LAUNCHING, instance_id=instance_id)
         log.info("slot_launched", extra={"slot_id": slot_id, "instance_id": instance_id})

agent/nix_builder_autoscaler/tests/test_runtime_ec2.py

@@ -73,7 +73,7 @@ class TestLaunchSpot:
         stubber.add_response("run_instances", response, expected_params)
         runtime = _make_runtime(stubber, ec2_client, config=config)
 
-        iid = runtime.launch_spot("slot001", "#!/bin/bash\necho hello")
+        iid = runtime.launch_instance("slot001", "#!/bin/bash\necho hello")
         assert iid == "i-12345678"
         stubber.assert_no_pending_responses()
 
@@ -90,8 +90,8 @@ class TestLaunchSpot:
             )
 
         runtime = _make_runtime(stubber, ec2_client, config=config)
-        runtime.launch_spot("slot001", "")
+        runtime.launch_instance("slot001", "")
-        runtime.launch_spot("slot002", "")
+        runtime.launch_instance("slot002", "")
         stubber.assert_no_pending_responses()
 
 
@@ -418,7 +418,7 @@ class TestErrorClassification:
         runtime = _make_runtime(stubber, ec2_client)
 
         with pytest.raises(RuntimeAdapterError) as exc_info:
-            runtime.launch_spot("slot001", "#!/bin/bash")
+            runtime.launch_instance("slot001", "#!/bin/bash")
         assert exc_info.value.category == "capacity_unavailable"
 
     @patch("nix_builder_autoscaler.runtime.ec2.time.sleep")
@@ -439,7 +439,7 @@ class TestErrorClassification:
         )
         runtime = _make_runtime(stubber, ec2_client)
 
-        iid = runtime.launch_spot("slot001", "#!/bin/bash")
+        iid = runtime.launch_instance("slot001", "#!/bin/bash")
         assert iid == "i-retry123"
         assert mock_sleep.called
         stubber.assert_no_pending_responses()
@@ -460,5 +460,5 @@ class TestErrorClassification:
         runtime = _make_runtime(stubber, ec2_client)
 
         with pytest.raises(RuntimeAdapterError) as exc_info:
-            runtime.launch_spot("slot001", "#!/bin/bash")
+            runtime.launch_instance("slot001", "#!/bin/bash")
         assert exc_info.value.category == "throttled"

agent/nix_builder_autoscaler/tests/test_runtime_fake.py

@@ -9,13 +9,13 @@ from nix_builder_autoscaler.runtime.fake import FakeRuntime
 class TestLaunchSpot:
     def test_returns_synthetic_instance_id(self):
         rt = FakeRuntime()
-        iid = rt.launch_spot("slot001", "#!/bin/bash\necho hello")
+        iid = rt.launch_instance("slot001", "#!/bin/bash\necho hello")
         assert iid.startswith("i-fake-")
         assert len(iid) > 10
 
     def test_instance_starts_pending(self):
         rt = FakeRuntime()
-        iid = rt.launch_spot("slot001", "")
+        iid = rt.launch_instance("slot001", "")
         info = rt.describe_instance(iid)
         assert info["state"] == "pending"
         assert info["tailscale_ip"] is None
@@ -24,7 +24,7 @@ class TestLaunchSpot:
 class TestTickProgression:
     def test_transitions_to_running_after_configured_ticks(self):
         rt = FakeRuntime(launch_latency_ticks=3, ip_delay_ticks=1)
-        iid = rt.launch_spot("slot001", "")
+        iid = rt.launch_instance("slot001", "")
 
         for _ in range(2):
             rt.tick()
@@ -35,7 +35,7 @@ class TestTickProgression:
 
     def test_tailscale_ip_appears_after_configured_delay(self):
         rt = FakeRuntime(launch_latency_ticks=2, ip_delay_ticks=2)
-        iid = rt.launch_spot("slot001", "")
+        iid = rt.launch_instance("slot001", "")
 
         for _ in range(2):
             rt.tick()
@@ -56,7 +56,7 @@ class TestInjectedFailure:
         rt = FakeRuntime()
         rt.inject_launch_failure("slot001")
         try:
-            rt.launch_spot("slot001", "")
+            rt.launch_instance("slot001", "")
             raise AssertionError("Should have raised")
         except RuntimeAdapterError as e:
             assert e.category == "capacity_unavailable"
@@ -65,16 +65,16 @@ class TestInjectedFailure:
         rt = FakeRuntime()
         rt.inject_launch_failure("slot001")
         with contextlib.suppress(RuntimeAdapterError):
-            rt.launch_spot("slot001", "")
+            rt.launch_instance("slot001", "")
         # Second call should succeed
-        iid = rt.launch_spot("slot001", "")
+        iid = rt.launch_instance("slot001", "")
         assert iid.startswith("i-fake-")
 
 
 class TestInjectedInterruption:
     def test_interruption_returns_terminated(self):
         rt = FakeRuntime(launch_latency_ticks=1)
-        iid = rt.launch_spot("slot001", "")
+        iid = rt.launch_instance("slot001", "")
         rt.tick()
         assert rt.describe_instance(iid)["state"] == "running"
 
@@ -85,7 +85,7 @@ class TestInjectedInterruption:
     def test_interruption_is_one_shot(self):
         """After the interruption fires, subsequent describes stay terminated."""
         rt = FakeRuntime(launch_latency_ticks=1)
-        iid = rt.launch_spot("slot001", "")
+        iid = rt.launch_instance("slot001", "")
         rt.tick()
         rt.inject_interruption(iid)
         rt.describe_instance(iid)  # consumes the injection
@@ -96,7 +96,7 @@ class TestInjectedInterruption:
 class TestTerminate:
     def test_terminate_marks_instance(self):
         rt = FakeRuntime(launch_latency_ticks=1)
-        iid = rt.launch_spot("slot001", "")
+        iid = rt.launch_instance("slot001", "")
         rt.tick()
         rt.terminate_instance(iid)
         assert rt.describe_instance(iid)["state"] == "terminated"
@@ -105,8 +105,8 @@ class TestTerminate:
 class TestListManaged:
     def test_lists_non_terminated(self):
         rt = FakeRuntime(launch_latency_ticks=1)
-        iid1 = rt.launch_spot("slot001", "")
+        iid1 = rt.launch_instance("slot001", "")
-        iid2 = rt.launch_spot("slot002", "")
+        iid2 = rt.launch_instance("slot002", "")
         rt.tick()
         rt.terminate_instance(iid1)
 

nix/modules/nixos/services/nix-builder-autoscaler.nix

@@ -66,7 +66,13 @@ in
       launchTemplateIdFile = lib.mkOption {
         type = lib.types.nullOr lib.types.str;
         default = null;
-        description = "Runtime file containing the EC2 launch template ID.";
+        description = "Runtime file containing the EC2 spot launch template ID.";
+      };
+
+      onDemandLaunchTemplateIdFile = lib.mkOption {
+        type = lib.types.nullOr lib.types.str;
+        default = null;
+        description = "Runtime file containing the EC2 on-demand launch template ID (required when capacity.nestedVirtualization is true).";
       };
 
       subnetIdsJsonFile = lib.mkOption {
@@ -216,6 +222,12 @@ in
         default = 1;
         description = "Launch batch size for the default system entry.";
       };
+
+      nestedVirtualization = lib.mkOption {
+        type = lib.types.bool;
+        default = false;
+        description = "Whether slots use on-demand instances with nested virtualization. Requires aws.onDemandLaunchTemplateIdFile to be set.";
+      };
     };
 
     security = {
@@ -256,6 +268,10 @@ in
         assertion = cfg.aws.subnetIdsJsonFile != null;
         message = "services.nix-builder-autoscaler.aws.subnetIdsJsonFile must be set.";
       }
+      {
+        assertion = !cfg.capacity.nestedVirtualization || cfg.aws.onDemandLaunchTemplateIdFile != null;
+        message = "services.nix-builder-autoscaler.aws.onDemandLaunchTemplateIdFile must be set when capacity.nestedVirtualization is true.";
+      }
     ];
 
     environment.systemPackages = [ cfg.package ];
@@ -301,6 +317,9 @@ in
                 install -d -m 0750 -o ${cfg.user} -g ${cfg.group} /run/nix-builder-autoscaler
                 launch_template_id="$(tr -d '\n' < ${lib.escapeShellArg cfg.aws.launchTemplateIdFile})"
                 subnet_ids_json="$(tr -d '\n' < ${lib.escapeShellArg cfg.aws.subnetIdsJsonFile})"
+                ${lib.optionalString (cfg.aws.onDemandLaunchTemplateIdFile != null) ''
+                  on_demand_launch_template_id="$(tr -d '\n' < ${lib.escapeShellArg cfg.aws.onDemandLaunchTemplateIdFile})"
+                ''}
 
                 cat > ${generatedConfigPath} <<EOF
         [server]
@@ -311,6 +330,9 @@ in
         [aws]
         region = "${cfg.aws.region}"
         launch_template_id = "$launch_template_id"
+        ${lib.optionalString (
+          cfg.aws.onDemandLaunchTemplateIdFile != null
+        ) ''on_demand_launch_template_id = "$on_demand_launch_template_id"''}
         subnet_ids = $subnet_ids_json
         security_group_ids = ${tomlStringList cfg.aws.securityGroupIds}
         instance_profile_arn = "${cfg.aws.instanceProfileArn}"
@@ -351,6 +373,7 @@ in
         launch_batch_size = ${toString cfg.capacity.launchBatchSize}
         scale_down_idle_seconds = ${toString cfg.capacity.idleScaleDownSeconds}
         termination_cooldown_seconds = ${toString cfg.capacity.terminationCooldownSeconds}
+        nested_virtualization = ${lib.boolToString cfg.capacity.nestedVirtualization}
         EOF
 
                 chown ${cfg.user}:${cfg.group} ${generatedConfigPath}